The present invention relates to multiple (or dual) phase alloys for hydrogen separation-purification and a method for the preparation of such alloys, as well as a metal membrane for hydrogen separation-purification and a method for the preparation of such a metal membrane.
In the preparation of various materials such as semiconductors, optical fibers and drugs, there has been used highly purified hydrogen gas and the amount thereof used therein has gradually been increased year by year. In addition, hydrogen gas has become of major interest lately as the fuel for a fuel cell. If such fuel cell is used in full-scale in the future, a considerable quantity of hydrogen gas would have to be supplied. For this reason, there has been desired for the development of a technique which permits the production of highly purified hydrogen gas in a large quantity at low cost.
As methods for preparing a large quantity of hydrogen gas, there have been proposed, for instance, (1) a method for preparing hydrogen gas which makes use of natural resources other than fossil ones or which comprises the step of the electrolysis of water and (2) a method which makes use of fossil natural resources or which comprises the step of reforming of hydrocarbons. As the electrolysis method (1), there has been investigated a method of electrolyzing water using electric energy obtained through the photovoltaic power generation or solar-electric power generation as an electric power source, but this method in the present technical status cannot be put to practical use. It would accordingly be practical that hydrogen gas is produced according to the method (2) or the steam reforming of hydrocarbons.
As has been described above, the reforming of hydrocarbons is suitable for the mass-production of hydrogen gas. For instance, in a reaction system comprising CH4 to which H2O has been added, a large amount of hydrogen gas is generated according to the following reaction scheme [reaction formulas (1) to (3)]:CH4+H2O ⇄CO+3H2[Gasification Reaction (endothermic reaction)]  (1)CO+H2O ⇄CO2+H2[Shift Reaction (exothermic reaction)]  (2)(1)+(2)=(3)CH4+2H2O ⇄CO2+4H2[Endothermic Reaction]  (3)
The reforming reaction takes place according to the reaction formulas (1) and (2) and the overall reforming reaction takes place according to the reaction formula (3). The reaction system includes contaminant gases such as CO, CO2, H2O and CH4 in addition to a large amount of hydrogen gas. To use hydrogen gas as a supply for a fuel cell, hydrogen gas should be purified and/or separated from these impurities. Moreover, the Pt electrode of such a fuel cell may be damaged unless the purified hydrogen gas has a CO content of not more than 10 ppm. In other words, the hydrogen gas should be purified to a high degree of purity before using the same in a fuel cell.
As methods for purifying hydrogen gas, there have been known, for instance, an absorption method, a cryogenic separation method or deep freezing processing method, an adsorption method and a membrane separation method. Among these, practically used is the membrane separation method. The membrane separation method makes use of the difference in the rate of membrane-permeation between various gases and the method employs a polymer film or a metal film as such a membrane.
In the membrane separation method using a polymer film, hydrogen gas is isolated and/or purified on the basis of the difference in the diffusion rate between gas molecules which pass through fine pores of the membrane or film. In this membrane separation method, it is difficult to provide hydrogen gas highly purified to a satisfactory level, but it permits the scaling up of the system for the preparation of purified hydrogen gas.
On the other hand, the metal membrane is almost free of any fine pore present in the polymer membrane and therefore, the hydrogen purification mechanism is as follows: If there is a difference in the hydrogen gas pressure between two compartments separated by a metal membrane, hydrogen molecules (H2) in the high pressure compartment are decomposed into elemental hydrogen or hydrogen atoms (H) on the metal membrane surface, and the resulting hydrogen atoms are then solubilized or dissolved into the metal membrane and undergo penetration and/or diffusion into the metal to thus form a solid solution. These hydrogen atoms permeate the metal membrane and are again combined into hydrogen molecules (H2) on the other surface of the metal membrane and emitted into the low pressure compartment and thus, hydrogen gas is purified. The hydrogen purification is significantly affected by the separation constant and permeability constant of the metal membrane. The purification which makes use of a metal membrane would permit the purification of hydrogen gas having a purity of about 99% into hydrogen gas having a purity of about 99.99999%. For this reason, it would be said that the membrane separation method using a metal membrane is suitably used for the preparation or purification of highly purified hydrogen gas which can be used in the fuel cell.
As hydrogen-permeable metal membrane capable of being used as hydrogen gas-separation membrane, there have been known, for instance, Pd-based alloys such as Pd—Ag alloys and Pd—Ti alloys (see, for instance, Japanese Un-Examined Patent Publication Hei 8-215551 (Section 0006 appearing on page 2)). Presently, there has been put on the market a Pd—Ag alloy membrane as a metal membrane for hydrogen separation-purification. If the fuel cell is used in full-scale and this accordingly requires the supply of a considerable quantity of hydrogen gas, the demand for such Pd—Ag alloy membrane would correspondingly be increased, which are used as the metal membrane for hydrogen separation-purification. Under such circumstances, one cannot take measures to meet the increasing demand for the Pd—Ag alloy foils since Pd is quite expensive and it is limited in the quantity as resources. Accordingly, it would be urgent to develop materials for such a metal membrane as a substitute for the Pd—Ag alloy membrane.
To highly efficiently use a metal membrane for the separation and/or purification of hydrogen gas, it would be quite important that the metal membrane has a high hydrogen-permeability and that the metal membrane can withstand the hydrogen gas pressure difference applied thereto or the membrane should have a high resistance to hydrogen embrittlement. In other words, the mechanical properties likewise serve as important factors. As a metal membrane serving as substitutes for the Pd—Ag alloy membrane, there have been investigated and developed V-M (M means a metal) solid solution single-phase alloys. However, in order to use the alloys as the metals for hydrogen-purification, the alloys should simultaneously satisfy the requirements for the hydrogen gas-permeability and for the resistance to hydrogen embrittlement. Accordingly, it would be impossible for a single metal and the above-mentioned solid solution single-phase alloy to simultaneously satisfy these two requirements.